Yes it is OK for HS wrestlers to COACH KIDS during the HS season (if they have the time/energy!). To do so will not "void" their eligibility.

However, if pay is involved you need to to be aware of the "play for pay" rules. KSHSAA Rule 21 deals with maintaining amateur status. The pertinent rules dealing with coaching/instructing are addressed in Section 1, Article 3(a),(b) and (c) of Rule 21 (page 33):

"Article 3: A student who receives pay for participation in athletics is ineligible in that sport under the provisions of this rule.
(a) A student may receive pay for teaching activities such as swimming, lifesaving, golf, tennis, wrestling, basketball, dancing, baseball, etc. provided he or she confines the work to teaching skills.
(b) A student may NOT receive pay for coaching teams or individual involved in competition.
(c) A student may receive pay for officiating athletic contests."